[SVG] invalidateRelativeLengthClients() should also clear resource caches

After r160730 we are no longer clearing resource container caches on
layout. Instead, we're relying on the layout invalidation path to handle
this task - and that includes SVGElement::invalidateRelativeLengthClients().

(This problem is only triggered under the mask/clip caching CL).

R=pdr@chromium.org,schenney@chromium.org

Review URL: https://codereview.chromium.org/49853003

git-svn-id: svn://svn.chromium.org/blink/trunk@160763 bbb929c8-8fbe-4397-9dbb-9b2b20218538